1. Field of the Invention
The present invention relates to hydraulic bending presses for deformation of sheet metal parts.
More specifically, the invention is concerned with bending presses of the type comprising a stationary upper platen and a movable lower platen actuated by at least three hydraulic jacks, two of which are placed at the lateral ends of the movable platen and at least one of which is placed at the center, these jacks being supplied from a single directional distribution system which controls the upward and downward movements of the movable platen.
2. Description of the Prior Art
Machines of this type have an advantage in that parallel deformation of the opposite edges of the two platens takes place during operation of the press by virtue of the fact that the thrust force exerted by the jacks on the movable lower platen is uniformly distributed along this movable platen in order to form constant and accurate folding angles. This machine geometry is that of machines described in French patent No. 1,362,471. In these machines, guiding means are provided for maintaining the movable platen parallel to the stationary upper platen during its displacements. As a rule, these guide means consist of rotating rollers carried by the movable platen and placed in contact with guide tracks provided on stationary elements of the frame. However, these mechanical guiding means prove to be insufficient in the event that the resultant of the forces generated by the work performed is not located in the axis of the machine. In such a case, the movable platen is in fact subjected to a tilting torque. Under these conditions, by reason of the elasticity of the different parts of the machine as well as the elasticity of the guiding means themselves, these means prove to be incapable of maintaining either general parallelism or parallelism of the opposite edges of the two platens as is required for ensuring angular precision along the entire length of the bend.
The problem of balancing of presses subjected to an eccentric work force has been a matter of concern to both manufacturers and users, whether in the case of plate-type presses for general use or bending presses having a long and narrow platen in which this problem is particularly critical.
The machine to which the invention applies in accordance with the present Application is of a very special type. In fact, it involves the use of a single pressure source, a single distribution for control of movements and at least three working jacks, at least one of which is placed at the center.
The different methods of balancing which have already been developed do not apply to this type of machine and to this configuration of elements. Even in the field of machines to which they do apply, they fail to obtain both a general parallel relationship or parallelism between the two platens and optimization of the parallelism of the opposite faces of the lower and upper platens as is exhibited by the machines in accordance with French patent No. 1,362,471 on condition that the load is centered, that is to say the parallelism which takes into account geometrical deformations of the platens under the action of work forces.
The techniques employed up to the present time are usually concerned with presses having two upper jacks and involve the use of a machine equipped with a unit for servo-distribution by means of a jack. In arrangements of this type, the pressure established within the jack is solely a function of the work reaction which takes place in opposition to the movement of the jack, the servo-distributor being intended to control a volume of oil supplied to the jack.
In devices equipped with two jacks, by reason of the very fact that the number of jacks is limited to two, the basic characteristic of parallel sag of the opposite platen faces, even under centered load conditions, cannot be realized as in the presses in accordance with French patent No. 1,362,471 which describes a technique involving the use of three jacks. Only general parallelism is ensured in all presses having two jacks and two servo-distributors.
When the load is off-center, these presses equipped with a device having two servo-distributors are subject to a disadvantage in that they are not always capable of controlling the deformations of the opposite faces.
Under eccentric load conditions, the use of servo-distribution control devices of the same type in a three-jack press would not make it easier to control the parallelism of deformation of the opposite edges. In fact, the pressure established within each jack is a function solely of the work force applied opposite to this jack. Under these conditions, there is no way of producing deliberate action on these pressures and therefore on the deformations.
U.S. Pat. No. 2,343,167 offers a different approach to the transmission of eccentric forces. In this patent, the general parallelism of the movable work platen is controlled in dependence on the position of a movable reference element. Since this reference element is not subjected to any work force, it completely sets aside the concept of parallel sags. Moreover, the hydraulic balancing device calls for a variable-throttling element which is preset by the operator during preliminary tests and is therefore not automatically adapted to requirements in order to overcome the eccentric load displacement. Aside from the fact that a pressure-limiting device has no effect in the static stage, that is to say in the stage of maintenance of pressure at the end of the working stroke which is an essential stage for the quality of shaping work, preadjustment is an all-or-none adjustment operation controlled by microswitches. These devices make it possible only to avoid roughly and non-automatically the major disturbances associated with an eccentric work force but in no way permit fine and automatic correction as is necessary in a hydraulic bending press.
The device described in French patent No. 2,545,418 is basically concerned with a search for optimization of parallelism of the opposite faces of the platens and clearly demonstrates the importance of optimization. In order to obtain this result, French patent No. 2,545,418 makes use of additional bearing points for one of the platens carried by auxiliary cross-members, in the same manner as French patent No. 2,347,992, and adds one or a number of supplementary jacks for bowing the other platen, the jacks themselves being carried by auxiliary cross-members.
These devices entail the need for a complex machine structure comprising three steel plates for constituting the stationary platen, three steel plates for constituting the movable platen, additional bearing points constituted by heavy steel pins and one or a number of additional jacks. This technique is cumbersome and costly. In addition, it is far removed from the geometry of machines of the type provided with a movable lower platen as described, for example, in French patent No. 1,362,471.
In the present state of the art, there does not exist any approach which makes it possible in machines having a minimum of three jacks to ensure at the same time general parallelism of the platens and optimization of parallelism of the opposite faces, whether the machine operates in a centered and/or eccentric load condition.
The present Application relates to a hydraulic circuit which makes it possible, while retaining this basic configuration of bending machines, to maintain general parallelism of the upper and lower platens as well as optimization of parallelism of the opposite faces of said platens, this being achieved under either centered or eccentric load conditions.
The object of the invention is to control the force exerted by each jack which produces action on the movable platen on the one hand in order to ensure general parallelism and on the other hand, by producing action on the force applied by each jack in respect of the same value of opposing torque, in order to generate a curve of deformation of the movable lower platen, thus ensuring optimum parallel deformation of the two platens.